Molecularly targeted imaging agents for the detection, staging, and monitoring of prostate cancer are urgently needed. Prostate Membrane Specific Antigen (PSMA) is a membrane glycoprotein expressed by all prostate cancers, and is the target of the antibody-based imaging agent ProstaScint. However, ProstaScint has not fulfilled its promise as a clinical or commercial imaging agent. We propose to employ protein engineering to produce a nanoscale biological targeting reagent based on a humanized PSMA-specific antibody. Engineered "minibodies" ( ~ 8-10 nm) are half the size of antibodies, yet retain full bivalent binding to their target, and are capable of rapid localization and imaging in vivo. Technical objectives are to: 1) produce PSMA-specific minibodies;2) radiolabel the biomolecules with residualizing (radiometal chelate) and non-residualizing (radioiodine) labels;3) evaluate cell binding and internalization in vitro;and 4) conduct microPET imaging studies in prostate cancer xenograft-bearing mice. Establishment of these technologies will create a pathway for generating imaging agents with any specificity. Public health relevance: studies will produce a novel imaging agent for specific detection of prostate cancer in patients with goal of commercialization. Success of these PSMA-specific minibodies will establish important groundwork for using thi s class of biological molecules for additional in vivo nanotargeting strategies.